1. Field of the Invention
The present invention relates generally to methods and apparatus for transmitting data in a computer network. More specifically, the present invention relates to methods and apparatus for providing constant bit rate (CBR) service over a time-slotted access channel.
2. Background
Broadband access technologies, such as cable, copper, fiber optic, and wireless have made rapid progress in recent years. Recently, there has been a convergence of voice and data networks which is due in-part to the U.S. deregulation of the telecommunications industry. In order to stay competitive, companies offering broadband access technologies need to support voice communication over their local access networks. For networks which use a shared access medium to communicate between subscribers and the service provider (e.g., cable networks, wireless networks, etc.), the support of high quality voice communication over such networks is not an easy task, as described in greater detail below.
One type of shared access network is a cable network or cable modem network, illustrated in FIG. 1 of the drawings.
FIG. 1 is a block diagram of a two-way hybrid fiber-coaxial (HFC) cable system utilizing a cable modem for data transmission. It shows a head end 102 (essentially a distribution hub) which can typically service about 40,000 homes. Head end 102 contains a cable modem termination system (CMTS) 104 that is needed when transmitting and receiving data using cable modems. Block 104 of FIG. 1 represents a cable modem termination system connected to a fiber node 108 by pairs of optical fibers 106. The primary functions of the CMTS are (1) receiving signals from external sources 100 and converting the format of those signals, e.g., microwave signals to electrical signals suitable for transmission over the cable system; (2) providing appropriate Media Access Control (MAC) level packet headers (as specified by the MCNS standard discussed below) for data received by the cable system, (3) modulating and demodulating the data to and from the cable system, and (4) converting the electrical signal in the CMTS to an optical signal for transmission over the optical lines to the fiber nodes.
Head end 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each head end can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the head end and each distribution node. In addition, because cable modems were not used, the head end of pre-HFC cable systems did not contain a CMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 110 to two-way amplifiers or duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service up to 500 subscribers. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
Recently, it has been contemplated that HFC cable systems could be used for two-way transmission of digital data. The data may be Internet data, digital audio, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide up to a thousand times faster transmission of digital data than is presently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 33,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data may be transferred at speeds up to 10 million bps. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
TABLE 1Time to Transmit a Single 500 kbyte ImageTelephone Modem (28.8 kbps)6-8 minutesISDN Line (64 kbps)1-1.5 minutesCable Modem (30 Mbps)1 second
Furthermore, subscribers can be fully connected twenty-four hours a day to services without interfering with cable television service or phone service. The cable modem, an improvement of a conventional PC data modem, provides this high speed connectivity and is, therefore, instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services.
As mentioned above, the cable industry has been upgrading its coaxial cable systems to HFC systems that utilize fiber optics to connect head ends to fiber nodes and, in some instances, to also use them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and faster than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers decrease a cable system's channel capacity, degrade the signal quality, and are susceptible to high maintenance costs. Thus, distribution systems that use fiber optics need fewer amplifiers to maintain better signal quality.
Digital data on the upstream and downstream channels is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert digital data to a modulated RF signal and convert the RF signal back to digital form. The conversion is done at two points: at the subscriber's home by a cable modem and by a CMTS located at the head end. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is fed to the cable modem which converts it to a modulated RF signal (it is helpful to keep in mind that the word “modem” is derived from modulator/demodulator). Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
Traditionally, voice was supported on telephone networks as a dedicated slot allocation in a circuit-switched connection. Such connections provide a dedicated channel for the entire duration of the call. However, this results in under-utilization of network resources. More recently, voice information may be transmitted across the cable network in a manner similar to that used for transmitting data across the cable network. Voice is being supported over IP on packet-switched networks where the voice is digitized, packetized, and shipped as IP frames over data networks. This results in higher utilization of bandwidth at the cost of delay and jitter experienced as these packets traverse through the data networks. There are a number of conventional standards for digitizing analog voice data, each trading efficiency, bandwidth, processing power, and quality. Such standards are commonly known to those skilled in the art.
One important difference between data communication and voice communication over digital computer networks is that voice service demands much more stringent standards for acceptable voice quality, particularly with respect to delay and jitter tolerances. Typically, most delay may be compensated for by buffering. However, for supporting toll quality voice, little or no jitter can be tolerated, particularly in the local access portion of the end-to-end Voice Over IP (VoIP) network. It is to be noted that, henceforth, the term “voice” refers to packetized voice data having stringent delay and jitter restrictions. A voice source can be abstracted as a source which generates constant size packets at a specific rate. Due to the range of standards that exist in the telecommunication industry today, a wide range of packet sizes and rates need to be supported by the cable network.
Shared Access Problems
In a HFC network, data is transferred by a request-grant mechanism. In order for a cable modem to transmit data to the Head End (CMTS), it must first send a data transmission request signal to the CMTS, informing the CMTS that it has data to send. The CMTS, on receiving a request from a cable modem, allocates one or more time slots on the upstream channel for that particular cable modem to send its data.
An upstream channel MAC scheduler, which is part of the CMTS, plays an important role in dictating remotely to the cable modems (via channel MAP messages) the predetermined use of time-slot assignments on the upstream channel. This scheduler works with several MAC/PHY/QoS constraints. A typical sequence of events for transferring data over a CATV system would be as follows:                (1) If the cable modem has data to transmit, it will send a time slot request to the CMTS.        (2) If the request reaches the CMTS successfully, it is queued depending upon the priority of the request. Otherwise, the cable modem retransmits its request after waiting a random amount of time.        (3) The CMTS will then allocate a time slice for this modem after satisfying all other higher priority requests and constraints.        (4) The cable modem, on receiving the grant transmits its data during its allotted time slice.        
This request-grant mechanism, though ideal for bursty traffic, is not acceptable for meeting toll quality voice requirements. For example, using the request-grant mechanism, delay and jitter become a function of other stations on the network, as well as a function of the load on the system. Additionally, the potential for large access delay jitter to the CMTS makes this scheme undesirable for real-time voice service. Further, the potential for jitter is also increased due to the fact that variable-length packets are supported by the CMTS.
While conventional data communication techniques for broadband access technologies are able to provide a minimum level of acceptable service quality for some of today's applications, there exists a continual need to improve data communication techniques for broadband access technologies so as to provide better quality service for supporting current and future applications which may include high-bandwidth applications such as voice, fax, video, etc.